1. Field of the Invention
The present invention relates generally to methods and apparatus for generating a plasma at or about one atmosphere, especially for purposes of treating various webs and films to enhance their properties, and to the treated webs and films, which have the improved and desirable properties.
2. Description of the Related Art
The surface treatment of polymer materials using a plasma discharge can lead to a broad range of improved results. A plasma discharge can be used to initiate chemical reactions on the surface of a substrate or roughen a surface from ion bombardment. One important benefit that can be achieved is to provide a more hydrophilic, or wettable surface. Plasmas produced under a vacuum have produced hydrophilic surfaces. However, this effect is typically short term for vacuum plasma treated materials. Recent experiments using a one atmosphere dielectric barrier discharge, with a sinusoidal excitation of a few kilohertz, have produced meltblown polypropylene samples which were wettable for eight months, and longer. However, the treatment times for these samples were generally on the order of four to five minutes, which is considered relatively long for practical applications.
By controlling certain processes of the plasma/substrate interaction, and by exploiting various features associated with a one atmosphere discharge, higher plasma power densities and shorter treatment times can be obtained. When exposed to a plasma, a substrate will be bombarded by electrons, ions, radicals, neutrals and ultraviolet (UV) radiation which is sometimes sufficient to cause sputtering or etching of the exposed surface. The resulting volatile products are likely to contaminate the working gas and can be redeposited on the substrate. Sufficient gas flow within the discharge zone can minimize these problems. However, in addition to etching and roughening the substrate, ions can react chemically with the substrate.
The energy and flux of ions to the substrate can be significantly increased by biasing the substrate, usually to a negative potential. Controlled substrate biasing for a high pressure discharge requires metal faced electrodes or an asymmetric voltage waveform when using a dielectric barrier discharge. A symmetric or sinusoidal waveform will alternately bias a substrate positively and then negatively throughout a cycle, partially reversing the effects produced by each half cycle.
The energetic UV radiation produced by a plasma can have a variety of effects on both the background gas and the polymer substrate. Vacuum UV (primarily at short wavelengths, typically 50 to 250 nm) can cause photoionization and bond dissociation yielding free radicals. Radicals produced on a polymer surface can cause crosslinking of a polymer chain or react with species present in the gas phase. For the production of a hydrophilic surface, oxygen or oxygen containing radicals must typically be present. Since many competing reactions will occur in an oxygen containing gas phase, and since these reactions will have temperature dependent reaction rates, proper control of the background gas temperature will result in higher concentrations of the appropriate species to enhance a given surface treatment.
Ultraviolet production in a gas phase discharge can be enhanced by the use of a gas with accessible emission lines (in the UV) for the operating mode of the discharge. Proper electrode geometry with metal faced electrodes reflective to UV, and a dielectric barrier transparent to UV, will enhance the UV levels in the gas discharge.